Dozens of people dressed in blue are being tied together with blue ribbon outside a north London Tube station. It doesn't take long for passers-by to start staring.
As the gathering of mostly strangers grows, one onlooker realises she, too, is wearing blue, and almost runs away lest she become entangled in the strange human web. The look on her face – something between bemusement and mild antipathy – is one that will become familiar over the next three hours. What on earth, she must be thinking, are we doing?
It's a question I'm asking myself and I know the answer. We have come to "be" art or, more specifically, a river. Most Londoners could name only the river, but dozens of waterways used to feed the Thames. Centuries of building and paving has consigned many, including the Fleet, the Effra, and the Tyburn, to the city's bowels. Others, including the Walbrook, which once bubbled and flowed through the oldest part of the capital, are no more than dry voids.
Amy Sharrocks, an artist fascinated by London's relationship with water, wants the forgotten Walbrook to run again – for an afternoon at least. She has charted the river's course from its source near Highbury and Islington Tube station, due south via the City to its mouth at the Thames near Cannon Street. Linked by ribbon bonds, like molecules of hydrogen and oxygen, a group of 50 participants will surge and meander along some of the capital's busiest pavements.
"We are tracing a memory of water, laying new stories over old histories, drafting and redrafting our footsteps across the city," Sharrocks says in a speech as the last volunteers are tied on. "Today there is no Walbrook. Until we get to the Thames, we are the water."
Time to open the floodgates. A fluid formation about 10 people deep and four people wide, we set off down Upper Street. As we move and chat, so the ribbons that suspend us shift and stretch. When people on the edge of the group move to avoid lamp-posts, bollards and pedestrians, the effect is that we flow past all obstacles. And when the rear half of the group slows at a pedestrian crossing, the ribbons force us to coalesce like water on oil. It's a comforting feeling, until the talk of imaginary water manifests itself as a very real – and urgent – need to go to the loo.
I set about finding Amy, who is nearer the head of the group. Another quirk of our bonds is the invisible currents and eddies they create. People drift almost imperceptibly upstream or downstream, moving around each other as we progress. And so, before long, I'm within chatting distance of Sharrocks. Straight away, I make a mistake by suggesting Walbrook, the name of this piece, is "a bit mad". Her infectious enthusiasm never wavering, she sets me straight. "It's a question of how you choose to spend your time," she says. For Sharrocks, 39, this is a normal afternoon. Walbrook is her third piece to explore London's links to water. In 2006, Drift saw her sharing an inflatable boat with individuals in a deserted south London swimming pool. The following year, SWim saw Sharrocks and others stroking across the capital using its pools, lidos and ponds, as well as an old bus, in a subversion of Burt Lancaster's solitary odyssey across America in the 1968 film The Swimmer.
"Water has so much to show us," says Sharrocks. "Other cities venerate it and its memory but London seems to have stamped it into channels, or even nothingness. The Thames is seen as something to defend against. Rather than explore our connection with water, we bulldoze through it. I want to break the barrier between soft and constantly moving water and the hard edges of the world we have built around it."
So that's why Sharrocks is here. I'm here to write this. But who are all these other people? Adrian Murphy, 29, has a professional interest in rivers – he works for the Royal Geographic Society. The quirkiness of Walbrook has clearly piqued his curiosity – he has brought as a companion a wind-up miniature yellow plastic duck called London. "It would be nice to set him free when we get to the Thames," he says, "but it's my friend's duck and I promised to give it back."
Anthony Melville is a lofty energy consultant who, like everyone here (except, perhaps, me) is "never worried about looking silly". He adds: "It tickled my imagination. I think it's a wonderful thing to liberate London's rivers and it's nice when art meets nature, especially when you can connect to it so directly."
Angie Zelter is an activist (of some renown: she has been arrested more than 100 times during protests). She admits she doesn't find today's walk very artistic but says she'll do anything for climate change. "Maybe when it comes and London is inundated, this will become a river again," says Zelter, 59. "You could say this is a warning."
Of all the blue clothes on show – mostly jeans and shirts or dresses – my favourite garment belongs to Simon Barrett, a retired librarian from Kent. A pair of faded blue suede brogues with pointy toes peak out from beneath his trousers. "I'm glad you like them," says Barrett, 65, before trying to explain why he's here. "I like walking and I guess I'm also intrigued. It's cutting edge; it's art; it's political. Hang on, I'm talking nonsense!"
You get the sense that Sharrocks revels in the confusion her art breeds in the minds of onlookers and participants. Hers may well be the most original work that London's forgotten rivers have inspired. But it isn't the first. "Rising Damp", by the late distinguished poet U A Fanthorpe, tells how "The little fervent underground Rivers of London... Whose names are disfigured, frayed or effaced... have gone under. Boxed, like the magician's assistant. Forgotten like the dead". But, as Fanthorpe later writes: "Being of our world, they will return... Will deluge cellars, detonate manholes, Plant effluent on our faces, Sink the city."
Fanthorpe, who died earlier this year, may not have envisaged the return of the Walbrook so soon (or without water), yet the feeling for those who have revived the river remains one of trickling through the city. Not that harried Londoners appear to appreciate it. Befuddlement peaks after we reach the subways beneath the swirling Old Street roundabout, where Sharrocks asks us to complete the walk in silence. ("Words can get in the way of feelings," she says.) "Eh, would ya look at this!" shouts a Scottish newspaper seller before he launches into a Glaswegian rendition of Sam Cooke's "Chain Gang". We say nothing.
As we approach Bank, where the first of the suits are clocking off for the weekend, a man leans out of a white van and offers: "Excuse me, you got somefink wrapped round ya." Then, at Moorgate, a City of London police van pulls up and deposits a bobby at the front of the procession, where Sharrocks is compelled to break her silence. "He kept asking what our intentions were," she recounts later. "It's a nice question to ponder but when he saw we posed no threat he left us to it."
So onwards we flow, passing St Stephen Walbrook, a church that once stood on the banks of the river. Finally we reach Cousin Lane, which leads to The Banker pub. In the final yards of our three-and-a-half mile course, we cascade down stone steps to a pebbly beach and the Thames. As Sharrocks unwinds the shimmering blue ribbon, she's already reflecting on her latest piece: "One thing that struck me was how much conversation took away from noticing other things in the first half, and how loud the city was when we were silent."
Sharrocks isn't the only one still mulling over Walbrook as we disperse. A cynic at the outset, I leave deciding there was something weirdly wonderful in its madness. (Sorry, Amy, it was a little bit mad, wasn't it?) Of the few onlookers who realised they were appreciating art, the simplest and perhaps best critique came about halfway along. As we ambled through a housing estate, a young couple walking their daughter home from school had to stand aside. The parents wore the same look of bafflement seen on the faces of the newspaper seller, the white-van man and the City suits. But their daughter, who was about eight and wore a blue checked dress, seemed to know exactly what was going on. Smiling, she said: "They're being a river."